The invention relates to a diffuser and, more particularly, to a diffuser for a centrifugal compressor which is configured to optimize flow distribution to a combustion chamber and which may be manufactured to close tolerances in a manner which assures uniformity between diffusers.
A centrifugal compressor includes a rotating impeller arranged to accelerate and thereby increase the kinetic energy of a gas flowing therethrough. The diffuser is generally characterized by a quasi-vaneless annular space surrounding the impeller. The diffuser acts to decrease the velocity of the gas flow leaving the impeller to transform the energy thereof to an increase in static pressure, thus generating pressurized gas.
Prior art diffusers have generally included a plurality of circumferentially spaced passages which converge to the annular space surrounding the impeller. These passages expand in area downstream of the impeller in order to diffuse the flow exiting the impeller. It has been found for prior art diffusers of this type which are to be utilized with gas turbine engines that it is preferable to have the diffuser passages assume an initial circular cross section so as to accommodate with minimal losses the relatively high-flow velocities of the gases exiting the impeller and thereafter gradually merge into a near-rectangular outlet to minimize losses.
One diffuser of this type is disclosed in U.S. Pat. No. 4,027,997 issued to A. C. Bryans on June 7, 1977, and assigned to the assignee of this invention. The Bryans diffuser comprises a plurality of linear passages in flow communication with a quasi-vaneless annular inlet surrounding the impeller of a centrifugal compressor. Each passage gradually merges from a circular cross section at a throat portion near its inlet end, to a near rectangular cross section at its outlet end defined by two flat opposing parallel sides and two flat opposing curved sides which produce a razor sharp trailing edge at the diffuser outlet. This near rectangular shape of the diffuser outlet optimizes the flow distribution to an annular combustion chamber in flow communication with the diffuser outlet.
Diffusers constructed in accordance with the teachings of the Bryans patent have demonstrated significant improvements in the performance of centrifugal compressors for gas turbine engines. However, inasmuch as the quasi-vaneless inlet of the diffuser receives accelerated gases directly from the impeller, it is subjet to relatively high viscous drag which results in undesirable pressure losses.
Accordingly, it is an object of the present invention to provide a new and improved diffuser for a centrifugal compressor.
Another object of the present invention is to provide a diffuser wherein the length of the quasi-vaneless inlet is reduced for reducing the total of viscous drag thereover.
Another object of the present invention is to provide a diffuser for a centrifugal compressor which is configured to optimize flow distribution to a combustion chamber and which may be easily manufactured to close tolerances in a manner which assures uniformity between diffusers.
SUMMARY OF THE INVENTION
The diffuser of the present invention comprises a plurality of passages which intersect at radially inner ends thereof to define a quasi-vaneless annular inlet for receiving accelerated gases from an impeller of a centrifugal compressor. Each of the passages includes a throat portion having a quadrilateral cross section, including two substantially parallel linear sidewalls and two substantially arcuate opposing sidewalls, effective for reducing the length of and thereby pressure losses from the annular inlet. The linearity and regularity of the diffuser passages enables the diffuser to be manufactured to close tolerances by electric discharge milling an annular plate utilizing a single tool. This assures uniformity and consistency between diffusers.